1. Field of the Invention
The present invention relates to a traffic control system for an asynchronous transfer mode (ATM) exchange which receives a description of parameters of traffic upon reception of a request-to-call connection from one calling subscriber terminal to another subscriber terminal.
Research and development for broad band integrated service digital networks (ISDN) have progressed to the point of providing a next generation communication network for a total communication service covering all ranges including voice, data, full motion video, and so forth. ATM is a technology for converting all data into cells with fixed packet length and for transferring the cells at a high speed, regardless of whether the data is continuous data such as a moving picture or the data is burst data, or without depending on the occurrence of the data or on the communication speed of the data.
In the ATM communication, however, because it handles, in an integrated form, voice, data, full motion video and so forth having a different bearer rate and burst characteristics, when traffic having strong burst characteristics exits, i.e., when the interval between continuous data sets varies, the variation of traffic in the communication route becomes large or, in other words, the dispersion of the communication speeds becomes large, thereby causing a discard of cells or delays which degrade the service quality. Therefore, a solution to this problem has been desired.
2. Description of the Related Art
ATM communication is performed by inserting a fixed length (53 bytes) packet, which is referred to as a cell, into a vacant time slot. The actual transfer speed may be changed by increasing or decreasing the number of cells. Each cell is constituted by a header and data. The header contains information such as the address of the destination.
As mentioned above, various types of information are transmitted through the ATM network. Namely, information requiring a high transfer speed (in case of full motion video, near 100 Mbps, where "bps" is bit per second) and information at low speed, such as voice, are multiplexed on the same transmission line and are transmitted. Therefore, various facilities can be employed as terminals and the respective characteristics thereof are different. The nature of the set-up calls may be one generating continuous information such as voice, or one intermittently generating information having strong burst characteristics, such as video, data, and so forth. These data are handled in an integrated form. Therefore, when traffic having strong burst characteristics is mixed in the communicating data, it becomes important to maintain the quality of service such as the cell loss rate or an end-to-end delay as required by the users.
An actual prior art of ATM communication cannot be presented because the ATM communication per se is a new technology, but it can be considered to use conventional traffic control technologies employed in the conventional circuit exchange and synchronous type packet exchange. Namely, in the case of a circuit exchange, a judgement is made whether or not the line is occupied by some subscribers. A call is rejected when the line is occupied by some subscribers and is accepted when the line is empty. On the other hand, in case of packet exchange, a judgement is made whether or not there is an empty (i.e., unused) capacity or vacancy, in the output route of the transmission line so as to allow communication when there is a vacancy.
On the other hand, there is a proposal in that, as a traffic control system in an ATM communication, once a call, which is requesting a call connection, is accepted, and when the communication is not performed in accordance with the parameters such as the peak speed described by the user, marking is provided on a part of the header for a cell which enters, with a band width (bit rate or speed) beyond the described band width, into a supervisory unit having a policing function. Then, if congestion occurs, the cell with the marking is discarded (see Electronic Communication Society Technical Study Material, SE87-137, P43-48).
In applying the technology of a packet switching system to ATM communication, one approach would be to accept a call when the sum of peak bands (peak speeds) of data described by calls can pass through the communication path. However, in such an approach, since other parameters of the call such as the average speed, burst period, idle period between bursts, and burst ratio are not considered in the call admission control, it is difficult to reduce the cell loss rate.
For example, when a call, which is requesting a call connection and has a high speed and strong burst characteristics, is accepted, and when a number of cells are set up at the same time, the total band width of one highway may become larger that the transmission ability, i.e., the transmission capacity, although it depends on characteristics such as the burst characteristics or the transmitting speeds of the calls which have been transferred through the same highway. In such case, cells of the requesting call are discarded without being transferred. Although such discarding of the cells can be avoided by expanding the capacity of the buffer in the network, it may, in turn, cause a delay which is undesirable for information, such as voice information, requiring real-time communication.
Conventionally, there is a problem in that it is difficult to accurately judge whether or not the required service quality such as the cell loss rate or delay can be maintained, upon admission of a call. Furthermore, there is a problem in that, once a call is accepted, it is difficult to maintain a predetermined level of service quality when congestion is caused by, for example, a call which has parameters beyond the described parameters.
On the other hand, according to the conventional method, in which the marking is put for a breaching cell which is transferred in the ATM switching system with a band width greater that the described band width after accepting the call, and the marked cells are discarded when congestion occurs, there is a problem in that the discard of cells may have a serious effect when the cells are for data transmission and so forth.